A thermal conductivity detector (TCD) may be employed in chromatography to detect components present in a sample gas. The device is particularly useful when placed at the output of a gas chromatograph (GC) where it can detect chemically distinct pulses of gas, called peaks, emerging at different times from the GC. In such an application, a TCD senses changes in the thermal conductivity of a column effluent comprising a sample gas which can be compared to a reference flow of a carrier gas, for example, helium. Since all compounds, organic and inorganic, have a thermal conductivity different from helium, all compounds can be detected by a TCD.
In general, a TCD consists of a thin electrically resistive filament in a temperature-controlled cell through which a gas (e.g., a column effluent) passes. In practice, a TCD monitors the thermal conductivity of the gas surrounding the thin electrically resistive filament by measuring the electrical power required to heat that filament to a given temperature. Gases with lower thermal conductivity require less power to heat the filament to a given temperature than gases with higher thermal conductivity. When an analyte elutes in a GC the thermal conductivity of the column effluent is reduced. In this case, if the column effluent is provided to the TCD, if the electrical power is kept constant then the filament heats up to a higher temperature and changes resistance. This resistance change may be sensed, for example, by a Wheatstone bridge circuit which produces a measurable voltage change.
FIG. 1 illustrates a Wheatstone bridge 10 employing TCDs 15a and 15b which can be employed in a gas chromatography system. A column effluent of a sample gas flows over resistor R3 in TCD 15a while a reference gas (e.g., helium) is passed over a second resistor R4 in TCD 15b in the four-resistor circuit. The reference gas flowing across resistor R4 of the circuit compensates for drift due to flow or temperature fluctuations. Changes in the thermal conductivity of the column effluent flow across resistor R3 will result in a temperature change of the resistor R3 and therefore a resistance change which can be measured as a signal. A described above, this temperature change will depend upon the gaseous compound currently flowing past the resistor, allowing that compound to be detected.
Two streams of development work have resulted in two different types of TCDs.
FIG. 2 illustrates a first type of prior art miniature thermal conductivity detector (TCD) 20, sometimes also referred to as a katharometer. TCD 20 includes a thin wire or filament 21 (e.g., tungsten or platinum), fixed on thicker metal posts 22 at two ends, the posts passing through electrical insulators 23 comprising, for example, glass. Wire 21 is suspended in a channel or cavity 24 formed in a substrate 25. A flowing gas whose properties are to be monitored is passed through channel 24, and filament 21 is employed as an electrically resistive filament analogous to a light bulb filament. If the gas medium surrounding wire 21 is mostly an inert medium such as helium, then filament 21 can be taken to a high temperature, for example 500° C., without burning out filament 21.
When filament 21 is at for example 500° C., substrate 25, for example comprising stainless steel, might typically be at 400° C. Typically, substrate 25 will have a higher coefficient of thermal expansion (CTE) than filament 21, and so filament 21 remains stretched taut even though it is at a higher temperature than substrate 25. If filament 21 failed to remain taut it could mechanically buckle, coming closer to the walls of channel 24, and its use as a detector would suffer. Of course if filament 21 were to stretch so tightly that it broke, then TCD 20 would become useless. Thus designing TCD 20 requires a balancing act between two CTEs, that of filament 21 and that of substrate 25.
Miniature TCDs such as TCD 20 have been successful, having utility over a high temperature range, so that they can characterize gases containing, for example, petroleum products that are liquid below 400° C. but gaseous at 400° C.
However, miniature TCDs require fairly large gas flow on the order of 20-50 standard cubic centimeters per minute (sccm) in order to give good accuracy. This high flow requirement is not well suited to the small-diameter capillary columns used in gas chromatography where the flow rates are on the order of 1-5 sccm. These low flow rates save gas and their use is desirable.
In order to take advantage of the existence of small-diameter capillary columns, a second stream of development produced microscale TCDs.
FIG. 3A illustrates one embodiment of a microscale TCD. TCD 30 includes: a first substrate 31; a filament 32; an electrically insulating film 33, and a second substrate 36. First and second substrates 31 and 36 together form a principle structure of TCD 30, and together define a cavity 37 in which filament 32 is, at least in part, disposed, and through which the sample gas whose properties are to be monitored can flow. Cavity 37 includes first and second troughs 34 and 35 with filament 32 suspended therebetween such that a column effluent comprising a sample gas flows around it, thereby allowing TCD 30 to detect the thermal conductivity of the gas.
However while microscale TCDs such as TCD 30 have been successfully employed at low temperatures, they have generally been unsuccessful at high temperatures. There are several reasons for this lack of success, among them being inadequate mechanical stability over a wide temperature range.
What is needed, therefore, is a microscale TCD providing both utility at low flow rates and mechanical stability over a wide temperature range.